1. Field of the Invention
The present invention relates to time base conversion apparatus, and more particularly relates to apparatus suitable for converting the time base of a digitized video signal.
2. Description of the Prior Art
It is known to use a video recording and reproducing apparatus to record and to play back an analog signal, such as an audio signal, which is converted into a digital signal. For this purpose, a so-called audio pulse-code-modulation (PCM) recorder is used for converting the analog audio signal to a PCM signal for recording by means of a video tape recorder (VTR) and for converting the digital signal played back from the VTR to analog form.
In the PCM device mentioned above, a time base converter is included to compress the time base of the digital signal so that synthetic synchronizing signals like those that normally accompany a video signal can be included therein prior to recording on the VTR. The time base converter also functions to expand the time base of the played back digital signal to restore the digital audio signal to its original form. For compressing the time base of the digital signal, data is written into a memory or memories at a low-speed clocking rate and is read out therefrom at a high-speed clocking rate. In a complementary fashion, for expanding the time base of the digital signal, the latter signal is written into the memory or memories at a high-frequency clocking rate, but is read out therefrom at a low-frequency clocking rate.
Accordingly, because the clocking rate used for writing into the memory is different from that used for reading out therefrom, the writing and reading operations are asynchronous with one another. For this reason, even though there may be no memory access request for reading out, a memory access request for writing in can take place. Particularly, in the case of time base expanding, the writing operation can overtake the reading operation, with the result that two or more sets of input data are overwritten in the same memory location, and the information to be written into the memory is lost.
Such time base compressing and expanding is also required in a so-called digital video tape recorder (DVTR) in which a video signal is digitized for recording and playback. In the case of a DVTR, a conversion of the signal format is carried out following the compression of the time base of the digital video signal prior to recording, and upon playback of the digitized video signal prior to expansion of the time base thereof. In the DVTR, compression and expansion are carried out even during the blanking portions between adjacent groups of data. Because these data groups in the reproduced digital video signal occur even during blanking portions which, in the analog video signal, do not provide picture information, but rather are normally associated with a synchronizing signal and a burst signal, the writing operation requires that memory access requests take place very often. As a result, the aforementioned overwriting problem can become quite troublesome in the reproducing of the digitized video signal.
Furthermore, a random access memory (RAM) is normally used as the memory for a time base converting apparatus. However, in the RAM the cycle time, that is the time required for a reading and/or writing operation, is generally long. As a result, if a reproduced digital video signal is processed in the RAM, delays between memory access requests are likely to occur. In other words, when the transmission rate of the reproduced digital video signal is fast, a second group of bits of the digital video signal is quite often reached before the next prior group of bits has been written into the RAM, with the result either that one of these groups of bits will be lost altogether, or that the one group of bits will be written over the next prior group of bits, thereby destroying the information from both groups.